FIELD OF THE INVENTION
The invention relates to a method and circuit configuration for determining a frictional value between a motor vehicle wheel and a road surface, wherein the vehicle speed, the angular acceleration of a wheel and a braking pressure are ascertained, a wheel slip is determined therefrom, and a frictional value is determined therefrom with an estimation algorithm. The method and the corresponding circuit serve to ascertain the current value of friction between a motor vehicle tire and the surface of a road. If this current frictional value is known accurately, then anti-lock brake systems (ABS) can be regulated especially precisely. For systems that provide traction control, anti-slip, four-wheel steering, driver information, and rough road detection as well, accurate knowledge of the dynamic friction can be useful.
A system for ascertaining the road adhesion between a tire and a road surface has been heretofore known from European published patent specification 0 323 066 B1. Sensors detect a road wheel turn angle, a steering force, and a vehicle speed. A processor processes the three sensor signals, and reads a coefficient of friction from a table, i.e. a road adhesion of the tire, associated with the three signals. Measuring the steering force is quite complicated. Moreover, the coefficient of friction also depends on other values besides the driving parameters measured by the three sensors.
In another prior art apparatus, as described in European published patent specification 0 312 096 B1, the coefficient of friction, which is dependent on the lateral acceleration dynamics of the vehicle, is calculated with a formula derived from force equilibrium at the wheels. To that end, the signals of sensors for lateral acceleration, steering angle of the front and rear wheels and vehicle speed, and the contents of a memory for stability factors such as the vehicle mass, the distances between the center of gravity and the front or rear axle, the curve forces on the front and rear wheels, and the yawing moment are evaluated. Once again, the effort and expense, especially for the sensors, are quite high and there is no provision for using the calculated coefficient of friction for anti-lock control.